The present invention relates to a tracking device for moving an optical pickup block or a magnetic head, for example, with use of a linear motor.
A conventional tracking device for the optical pickup block is shown in FIG. 6. An optical pickup block 1 having an optical pickup is supported by a pair of parallel feed shafts 2 and 3 which are inserted through the block and extend in a radial direction of a compact disc, not shown. The optical pickup block 1 is slid on the feed shafts 2 and 3 in a predetermined range from an inner circumferential side of the compact disc to an outer circumferential side thereof. There are provided at both ends of the feed shafts 2 and 3 block stoppers 4 and 5 for stopping excess movement of the optical pickup block 1. Further, there are provided a linear motor 6 and a speed sensor 7 on opposing sides of the block.
The linear motor 6 comprises a yoke 8, a pair of magnets 9 and 10, a coil bobbin 11 and a driving coil 12. The yoke 8 is formed by an outer yoke portion 8a, a center yoke portion 8b and an inner yoke portion 8c, and extends in parallel relationship with the feed shafts 2 and 3. The magnets 9 and 10 are bonded to the outer yoke portion 8a and the inner yoke portion 8c, respectively, in such a manner as to be opposed to the center yoke portion 8b as shown. The coil bobbin 11 around which the driving coil 12 is wound is fixed at one end to a side of the optical pickup block 1, and is designed to freely slide on the center yoke portion 8b.
The speed sensor 7 comprises a yoke 13, a magnet 14, a coil bobbin 16 and a detection coil 17. The yoke 13 is formed by an outer yoke portion 13a and an inner yoke portion 13b. The magnet 14 is bonded to the outer yoke portion 13a as shown. The coil bobbin 16 is designed to freely slide on the inner yoke portion 13b, and is fixed at one end to an opposing side of the optical pickup block 1 in almost the same manner as with the linear motor 6.
In the aforementioned feed device, uniform magnetic fields are formed between the center yoke portion 8b of the linear motor 6 and the magnets 9 and 10. When a driving current is applied to the driving coil 12, the linear motor 6 is driven by an electromagnetic force, to thereby apply a tracking force to the optical pickup block 1. Further, a uniform magnetic field is also formed between the inner yoke portion 13b and the magnet 14, and when the optical pickup block 1 is moved a voltage is induced in the detection coil 17. Thus, the speed of the optical pickup block 1 is detected from such an induced voltage, and the speed detected is fed-back to the linear motor 6.
This kind of tracking device for the optical pickup block is disclosed in Japanese Utility Model Laid-open No. 60-47164.
However, in the conventional tracking device, when the optical pickup block 1 is stopped at a predetermined position, a high frequency component of the signal from the driving coil 12 is fed-back to the speed sensor 7. As a result, a magnetic field H is generated having a direction which is always inverted, which magnetic force H affects the detection coil 17 to induce a noise voltage. Therefore, a noise component is undesirably detected. That is, the detection coil 17 erroneously detects the noise component due to the magnetic field H, thus causing a detection error and interfering with detection of the speed signal of the optical pickup block 1.
In order to make the device compact and lightweight, it may be proposed that a common magnet and a common yoke should be used and the linear motor 6, the speed sensor 7, the driving coil 12 and the detection coil 17 should all be fixed on the same side of the optical pickup block 1. However, the influence of the magnetic field H would become pronounced, greater in this arrangement, thus increasing the noise component. Therefore, this arrangement is not practical.